1. Field of the Invention
The present invention concerns a portable motorized screwdriver or "electric" screwdriver.
2. Description of the Prior Art
This type of screwdriver is well known and generally comprises, on a casing designed to be held in the hand, a rotatable tool-bearing spindle that a usually electric motor housed within the casing drives in rotation in a selected screwing or unscrewing direction; to this end there are provided on the casing manually operated switching means for selecting one or other of these directions of rotation and starting and stopping the motor.
To enable the user of this type of screwdriver to further tighten a screw by applying to it a screwing torque greater than that which the motor can provide or to start unscrewing a screw by applying to it an unscrewing torque also greater than the torque that the motor can provide, without the user having to employ a manual screwdriver for this purpose, it has already been proposed in the prior art to provide such motorized screwdrivers with means for locking the spindle against rotation relative to the casing; to this end U.S. Pat. No. 3,802,518 provides a freewheel device between the motor and spindle, the device being adapted to be actuated by manual translation of a sleeve external to the casing to immobilize the spindle against rotation in one direction or the other, at will; U.S. Pat. No. 4,078,589 and European patent application No. 0,118,215 describe a provision for locking the spindle against rotation by means of a pin operated from outside the casing by manually actuating a button or sleeve, with no possibility of choosing the immobilization direction.
Although the various methods of locking the spindle against rotation proposed in the prior art make effective provision for using the motorized screwdriver as a conventional manual screwdriver, which makes any such motorized screwdriver more convenient to use, they nevertheless have a serious disadvantage in that they are totally dissociated from the switching means used to start and stop the motor; because of this, there is mothing to prevent the motor being started with the spindle locked against rotation, with the serious risk, vitually a certainty, of seriously damaging the motor.
An object of the present invention is to eliminate this risk.